Why did patent ductus arteriosus commonly left open?

Confusing question: A PDA is an energy saving bypass used in the developing fetus to send most of the hearts blood flow to the body rather than the lungs before birth. After birth, the rise in blood oxygen content usually triggers the bypass to close.It remains open in some cases & can create problems in later life. Closure by medicine, catheter or surgery is the common practice in the us by school age.

Not left open: The PDA will generally close on its own within the first couple days of life. If it does not close on its own and it is big enough to be audible with the stethoscope, then we generally close it at some point in childhood. If it is big enough to cause problems in infancy, then we close it sooner, usually with surgery. Sometimes it is kept open temporarily in complex congenital heart disease.